In U.S. patent 2006/0006986, a self-test method is proposed for a known RFID read/write station, which self-test method uses a so-called check-tag system to obtain a meaningful test result. At the same time, an additional circuit unit is mounted onto the RFID read-write station (hereinafter also called a read station, for short), which produces a signal that is at least similar to that of a back-scattered signal response from a tag and, as known for a modem, is supplied in the loopback method of the receiver, to be precise, by means of its own antenna or a coupler to receiver input of the read station. A disadvantage appears in this device, in that the self-test has to be deliberately triggered at the time by the user or a self-test program, and the read/write operation must be interrupted to do this. Furthermore, the one tag emulating a circuit accordingly is expensive and associated with related material costs, particularly when a genuine tag is to be emulated as precisely as possible. The prior art mentioned also allows for monitoring eight cable connections or antennas outside, which, in the operation of, several antennas locally set up at the read stations thus cuts down more severely on the load.
U.S. Pat. No. 6,486,769 describes programmable check tags, which are disposed in the area of tags to be read for the purpose of regulating transmission power and for diagnostic purposes, with the intent to adjust the transmission output of the read station so that tags are read in a first zone and not read in a second or more distant zones. Moreover, in this procedure, the output values determined for diagnostics are compared with expected values. This procedure may be found to be disadvantageous on the other hand in not being simultaneous with reading the tags to be read in operation and may be controlled as special routines by the processor, in which all the tags found in the field outside the check tags are placed in an inactive status.
A further disadvantage in some applications is the placement of check tags in the field for long-term use. In order to ensure functioning, the check tag and its surroundings must not be altered relative to propagation properties during the period of operation, either covered by materials or only shifted or moved by persons. It is known to the specialist that the range limits of radio frequencies in the UHF and microwave region cannot be fully defined in extent geometrically in actual environments. Rather, it all depends on calculating reflections over distance of a transmitted signal with a non-constant signal-strength path comparable to a corrugated sheet of metal with distance. No defined coupling exists between the check tags and the read equipment. The method here is well suited to a manual rather than to a permanent automated system test (continuous monitoring). A further disadvantage of the external check tags can be seen, in that, when installing RFID read-write stations, the placement site for the antennas and the check tags is often not yet known precisely and thus a direct operational test of the system as an aid to installation is not possible.
A problem consists of the fact that in a more complex RFID read-write station, several antennas are generally hooked up to read-write equipment, connected by coaxial cables, and thus a meaningful self-test, periodic or aperiodic, turns out to be very expensive and time-intensive for each or for one read-write process, which involves all the station components. Additional circuits and control software are required, which make the testing of transmitters and receivers possible.
A meaningful automated self-test of the antennas and their cabling is, then, aside from measuring the coefficients of reflection, not achievable with simple and cost-effective measures, because the effect of the signal quality on a tag and on its back-scattered response signal is complex and is not picked up. Also, no comparison is possible with previous results, because as a rule, the same tag only appears once or a few times at a specific read station.
The level of interference signals or their spectral distribution can be determined in the receiver, but not its effect on the quality of data monitoring to and from a tag. In particular, no information is obtained on the direct effect of interferences originating outside the RFID application on the operational channel used or of its neighboring channels upon the quality of the read/write data transmitted during operation. Also, in the so-called dense-reader mode, with more closely packed RFID read/write stations, the deterioration in quality of the results read due to the many carrier signals sent simultaneously is not directly measurable. In exactly the same way, the correct allocation of antennas set up at an operational site is not automatically verified after installation has occurred.